1. Field of the Invention
The present invention relates to a fluorescent lamp lighting apparatus. More particularly, the present invention relates to a lighting apparatus for driving a fluorescent lamp by using a plurality of switching devices.
2. Description of the Related Art
A so-called "series inverter" shown in FIG. 6 is one example of an exemplary prior art fluorescent lamp lighting apparatus. In the circuit shown in FIG. 6, before a fluorescent lamp 8 is started, current flows from an alternating current (AC) power supply 1 through a rectifier circuit 3. The AC voltage is rectified by the rectifier circuit 3. Thereafter, the voltage is not only charged in a ripple filtering capacitor 4, but also flows through a resistor 11, a resistor 23 and a secondary winding 25c of a current transformer 25 so as to be charged in a trigger capacitor 14. When the voltage of the trigger capacitor 14 reaches the gate threshold voltage of an FET 6, the charge accumulated in the trigger capacitor 14 is applied to the gate of the FET 6 so that the FET 6 is turned ON.
Once the FET 6 is turned ON, the current flows from the AC power supply 1 through the rectifier circuit 3, a resonance capacitor 7, one electrode 8a of the fluorescent lamp 8, a preheat capacitor 9, the other electrode 8b of the fluorescent lamp 8, an inductor 24, a primary winding 25b of the current transformer 25 and the drain of the FET 6, while increasing the amount thereof. Then, the current passes through the rectifier circuit 3 again to return to the AC power supply 1.
Then, a voltage is generated in a secondary winding 25c of the current transformer 25 by the current flowing through the primary winding 25b of the current transformer 25, thereby applying a gate voltage to the FET 6 and keeping the FET 6 turned ON. In the meantime, though the current flowing through the primary winding 25b of the current transformer 25 continuously increases, the magnetic saturation of the core of the current transformer 25 occurs at a certain point in time, when the current transformer 25 loses the function as an inductance.
When the core of the current transformer 25 is magnetically saturated, voltage is no longer output from the secondary winding 25c of the current transformer 25. As a result, the voltage applied between the gate and the source of the FET 6 decreases to the gate threshold voltage thereof or lower, so that the FET 6 is turned OFF.
It should be understood that the output polarity of the secondary winding of the current transformer 25 is inverted when the core of the current transformer 25 is magnetically saturated, because the current caused by the energy stored in the current transformer 25 is current having a variable amount dependent upon time. That is to say, since the voltage applied between the gate and the source of an FET 5 increases, the FET 5 is turned ON soon.
Once the FET 5 is turned ON, the current flows through the resonance capacitor 7, the FET 5, the primary winding 25b of the current transformer 25, the inductor 24, one electrode 8b of the fluorescent lamp 8, the preheat capacitor 9 and the other electrode 8b of the fluorescent lamp 8 so as to return to the resonance capacitor 7. This current resonates in a closed circuit including the resonance capacitor 7, the FET 5, the current transformer 25, the inductor 24, one electrode 8b of the fluorescent lamp 8, the preheat capacitor 9 and the other electrode 8b of the fluorescent lamp 8.
When the core of the current transformer 25 is magnetically saturated again owing to the inversion of the current, voltage is no longer output from a secondary winding 25a of the current transformer 25. As a result, the voltage applied between the gate and the source of the FET 5 decreases to the gate threshold voltage thereof or lower so that the FET 5 is turned OFF. Subsequently, when the output polarity of the secondary winding of the current transformer 25 is inverted, the FET 6 is turned ON again. Such an operation is repeatedly performed thereafter.
The current flows through the electrodes 8a and 8b of the fluorescent lamp 8, thereby heating these electrodes 8a and 8b. In addition, since a voltage having a large amplitude owing to the resonance is simultaneously applied to the electrodes of the fluorescent lamp 8, the temperature of the electrodes rises, thereby lighting the fluorescent lamp 8.
As described above, a prior art fluorescent lamp lighting apparatus uses a current transformer for switching the fluorescent lamp at a radio frequency. However, the use of a current transformer for such a prior art fluorescent lamp lighting apparatus prevents such an apparatus from being downsized. Moreover, since a current transformer is expensive, the fabrication costs of such a lighting apparatus are disadvantageously high.